A conventional printing head section in a spring-charged dot printer is shown in FIGS. 1 and 2. In these figures, reference numeral 1 designates comb-shaped yokes with teeth la on which deenergizing or demagnetizing coils 4 are wound; and reference numeral 2 designates a permanent magnet held between the two base parts lb of the two yokes 1. In the printing head section, first end portions 5a of leaf springs 5, which include respective printing hammers 6, are attracted to the front ends 1c of the teeth 1a of the yokes 1. The other end portions 5b of the leaf springs 5 are integral with a common base plate 8. The base plate 8 is held by means of fixing screws 10 between a retaining plate 7 and the mounting surface 11 of the lower part of a U-shaped support 3. The two yokes 1, the permanent magnet 2 and the coils 4 are sealingly fixed inside the support 3 with the front ends of the yoke teeth exposed. The support 3 is made of resin or the like.
The leaf spring mounting surface 11 of the support is inclined so that a large spring force is obtained when the upper end portions 5a of the leaf springs 5 are attracted to the attracting surfaces 1c of the front ends of the yoke teeth.
Mounting holes 12 are used in conjunction with the fixing screws 10 to fixedly secure the base plate 8 of the leaf springs 5 between the retaining plate 7 and the support 3. The fixing screws 10 are mounted so as to be perpendicular to the leaf spring fixing surface 11 of the support 3. The screws 10 are inserted into mounting holes 7a formed in the retaining plate 7 and mounting holes 5d formed in the lower end portions 5b of the leaf springs 5, and are then screwed into the mounting holes 12 in the leaf spring fixing surface.
Accordingly, the mounting holes 12 must be inclined with respect to the bottom 13 of the support. In the case where the support 3 is molded with resin, parts of the mold for forming the external surfaces, such as for instance the bottom, are different in the mold drawing direction from the part of the mold for forming the mounting holes 12. Accordingly, the mold is intricate in construction and costly to manufacture. Furthermore, the mounting members such as the screws 10 must be inclined with respect to the bottom 13 of the support 3 during assembly of the printing head section. Thus, the efficiency in manufacturing the conventional printing head fixing device is low.
Furthermore, the conventional spring-charged dot printer has a leaf spring 5 as shown in FIG. 7. The free end portions 5a of a leaf spring 5, which have printing hammers 6, respectively, are attracted to the teeth 1a of the yokes. The leaf spring 5 is comb-shaped and has a plurality of teeth 5c extending from its fixing part 5b. The fixing part 5b is held between the support 3 and a clamp plate 7 with fixing screws 10. As is apparent from the above description, the teeth 5c of the leaf spring are held on the yoke surface by the magnetic force of the permanent magnet 2. However, when current is applied to the demagnetizing coils 4, the teeth 5c are released from the yoke surface, as a result of which strain energy is produced in the leaf spring, thus causing the printing hammers 6 to strike a printing surface to print characters.
In this operation, in order to allow the teeth 5c to print characters with the same density, the teeth 5c should be uniformly held with respect to elastic energy. Accordingly, it is necessary for the teeth of the leaf spring to be of uniform configuration. Furthermore, in fixing the leaf spring, it is also necessary that the fixing part 5b be fixedly secured in such a manner that the conditions of the teeth with respect to the fixing part 5b are identical. When the leaf spring 5 is tightened through the clamp plate 7 with the fixing screws 10, the tightening pressure near the fixing screws is higher, and thus the tightening pressure at the fixing part 5b of the leaf spring is not uniformly, applied to the free ends 5a of the teeth 5c. In FIG. 7, the shaded part A indicates a high pressure distribution provided when the leaf spring is tightened through the clamp plate 7 with the fixing screws 10. Because of the tightening screw holes 5d, the free ends 5a of the teeth 5c of the leaf spring 5 are not uniformly tightened, and accordingly characters are printed non-uniformly.